Distributed optical sensing systems are optoelectronic devices which measure the primary quantity, e.g. a temperature, by means of optical waveguides such as fibers functioning as linear sensors. For example, it is known to use the Raman effect to determine the temperature along the fiber which can be of several kilometers in length.
An article of D. Hwang et al., “Novel auto-correlation method in a fiber-optic distributed-temperature sensor using reflected AntiStokes Raman scattering”, Optics Express 2010, Vo. 18, No. 10, p. 9747 ff. describes a method for auto-correction of fiber optic distributed temperature sensor using AntiStokes Raman back-scattering and its reflected signal is presented. This method processes two parts of measured signal. One part is the normal back scattered AntiStokes signal and the other part is the reflected signal which eliminate not only the effect of local losses due to the micro-bending or damages on fiber but also the differential attenuation. Because the beams of the same wavelength are used to cancel out the local variance in transmission medium there is no differential attenuation inherently. The auto correction concept was verified by the bending experiment on different bending points.
An article of Marcelo A. Soto et al., “High-Performance Raman-Based Distributed Fiber-Optic Sensing Under a Loop Scheme Using Anti-Stokes Light Only”, IEEE Photonics Technology Letters, Vol. 23, No. 9, 2011. Both ends of a fiber have been connected to a sensor through a 1×2 optical switch, allowing pulses to be alternately sent in both forward and backward directions. The geometric averages of the normalized AntiStokes traces in both forward and backward directions are calculated. The loop scheme is described as being advantageous over a mirror scheme where the light pulses are reflected by a mirror due to the two-way optical path in the mirror scheme, but the rear end of the fiber must be accessible by the sensor.
WO 2006/045340 A1 relates to measuring a distributed physical property (T(x)) of an optical device under test (DUT). Therefore, a probing signal comprising a sequence of optical pulses at a transmission wavelength is launched into the DUT, a corresponding optical response returning from the DUT is detected and at least a first response signal at a first response Q wavelength range is separated from the optical response, wherein the first response wavelength range does not comprise the transmission wavelength, a first correlation function is determined y correlating the first response signal and the probing signal and the distributed physical property is determined on the base of the first correlation function.
The described method may further comprise separating a second response function at a second response wavelength and determining a second correlation function by correlating the second response signal and the probing signal, and determining the distributed physical property on the base of the first correlation function and the second correlation function.
EP 0 692 705 B1 discloses a method for evaluating optically backscattered signals for determining a temperature profile of a backscattering medium. The light of a light source is modulated in its amplitude with respect to time. The signals evaluated are subjected to Fourier transformation.
EP 0 300 529 A1 discloses a method of measuring temperature which comprises launching input pulses of light into a temperature sensing element and deriving the temperature at a position in the element from the intensity of light scattered at said position, a part of the element being maintained at a known temperature in order to provide a reference for deriving temperature measurements at other positions in the element, thereby to avoid difficulties with calibration of the apparatus used to carry out the method.
GB 2 400 906 A discloses a method of obtaining a distributed measurement which comprises deploying an optical fibre in a measurement region of interest, and launching into it a first optical signal at a first wavelength and at a high power level, a second optical signal at a second wavelength, and a third optical signal at the first wavelength and at a low power level. These optical signals generate backscattered light at the second wavelength arising from Raman scattering of the first optical signal which is indicative of a parameter to be measured, at the first wavelength arising from Rayleigh scattering of the first optical signal, at the second wavelength arising from Rayleigh scattering of the second optical signal, and at the first wavelength arising from Rayleigh scattering of the third optical signal. The backscattered light is detected to generate four output signals, and a final output signal is derived by normalizing the Raman scattering signal to a function derived from the three Rayleigh scattering signals, which removes the effects of wavelength-dependent and nonlinear loss.
WO 2009/092436 A1 relates to distributed temperature sensing using two wavelengths differing by Raman shift of a waveguide. In contrast to conventional single input wavelength approaches in which beams related to a Stokes line and an anti-Stokes line experience different attenuation and propagation velocity of the corresponding electromagnetic radiation beam (in time and/or in space) when traveling to a detector, exemplary embodiments apply sequential stimulus signals having frequencies f1 and f2, whereas f2−f1 is approximately v, v being the Raman shift of the material. A result is that the detector “sees” essentially the same attenuation and propagation velocity for both measurements, and fiber attenuation and dispersion effects can be cancelled out at least partially.